MAX8520 MAXIM [Maxim Integrated Products], MAX8520 Datasheet - Page 13

• Manufacturer Part Number: MAX8520
• Description: Smallest TEC Power Drivers for Optical Modules
• Manufacturer: MAXIM [Maxim Integrated Products]

The MAX8520/MAX8521 dual buck converters operate
in phase and in complementary mode to drive the TEC
differentially in a current-mode control scheme. At zero
TEC current, the differential voltage is zero; hence, the
outputs with respect to GND are equal to half of V
As the TEC current demand increases, one output goes
up and the other goes down from the initial point of
0.5V
✕
buck converter depends on the operating I
R
and cooling are identical, but reverse in polarity, the
calculations only need to be carried out for either one.
For a given inductor and input voltage, the maximum
inductor ripple current happens when the duty cycle is
at 50%. Therefore, the inductor should be calculated at
50% duty cycle to find the maximum ripple current. The
maximum desired ripple current of a typical standard
buck converter is in the range of 20% to 40% of the
maximum load. The higher the value of the inductor, the
lower the ripple current. However, the size is physically
larger. For the TEC driver, the thermal loop is inherently
slow, so the inductor can be larger for lower ripple
current for better noise and EMI performance. Picking
an inductor to yield ripple current of 10% to 20% of the
maximum TEC current is a good starting point.
Figure 3. Functional Diagram of the Current-Control Loop
TEC
R
TEC
DD
. Since inductor current calculations for heating
). Therefore, the operating duty cycle of each
by an amount equal to 0.5
CTLI
1
______________________________________________________________________________________
0.5X
R
Inductor Selection
R
REF
✕
V
TEC
Smallest TEC Power Drivers for
gm
C
(V
COMP
TEC
1/4 V
REF
TEC
COMP
DD
= I
and
TEC
DD
-1.2
+1.2
.
1.2X
Calculate the inductor value as follows:
where LIR is the selected inductor ripple-current ratio,
I
switching frequency.
As an example, for V
1MHz, L = 4.58µH.
Even though each inductor ripple current is at its maxi-
mum at 50% duty cycle (zero TEC current), the ripple
cancels differentially because each is equal and in
phase.
The common-mode filter capacitors (C2 and C7 of Figure
1) are used as filter capacitors to ground for each output.
The output ripple voltage depends on the capacitance,
the ESR of these capacitors, and the inductor ripple
current. Ceramic capacitors are recommended for their
low ESR and impedance at high frequency.
TEC(MAX)
10X
CS
OS1
PWM
is the maximum TEC current, and fs is the
4X
LX2
LX1
Common-Mode Filter Capacitors
3/4 V
L
Optical Modules
=
Output Filter Capacitor
DD
LIR
DD
(
0 25
×
.
= 3.3V, LIR = 12%, and fs =
I
TEC MAX
×
(
R
V
SENSE
DD
)
)
×
fs
Selection
13